Testing circuits for echo suppressors



Nov. 10, 1925. 1,560,544

G. CRISSON TESTING CIRCUITS FOR ECHO SUPPRESSORS Filed Oct. 2 1925'INVENTOR 6'. ()155012 ATTORNEY Patented Nov. 10, 1925.

UNITED STATES PATENT OFFICE.

GEORGE CBISSON, OF HACKENSACK, NEW JERSEY, ASSIGNOR TO AMERICAN TELE-PHONE AND TELEGRAPH COMPANY, A CORPORATION OF NEW YORK.

TESTING CIRCUITS FOR ECHO SUPPRESSOBS.

Application filed October 25, 1923. Serial No. 670,741.

To all whom it may concern.

Be it known that I, Gronon GRISSON, residing at Hackensack, in thecounty of Bergen and State of New Jersey, have invented certainImprovements in Testing Circuits for Echo Suppressors, of which thefollowing is a specification.

This invention relates to echo suppressing arrangements for longtelephone circuits and more particularly to a method and means fortesting the operating time of echo suppressor circuits.

In long telephone circuits, particularly circuits of the four-wire type,it has been proposed to overcome the difficulty due to echoes which arereflected back from terminal points or points of impedanceirregularities in the circuit by providing so-called echo suppressorswhich operate inresponse to talking currents transmitted in onedirection to disable the circut for transmission in the oppositedirection. In the case of the four-wire circuit, for example, a voiceoperated relay may be arranged to operate in response to voice currentstransmitted in one direction to short circuit the line transmitting inthe opposite direction. The operating time of the echo suppressor insuch a system is of importance for the reason that when the suppressoris actuated in response to voice currents it must not release until thelast wave which has actuated the suppressor has had time to betransmitted to the terminal and back to the point at which thesuppressor short circuits the return path.

In accordance with the present invention it is proposed to test theoperation of the echo suppressor by applying an alternating current tothe suppressor arrangement of such frequency and amplitude as toapproximate the effect of voice currents. A switching device is providedwhich interrupts the alternating current and at the sametime switches acondenser into circuit with the contact of the suppressor relay so thatthe condenser is being charged up during the interval that thesuppressor relay holds over after the alternating current is interrupted, the potential to which the condenser is charged is determinedby balancing it against a known potential. The time that the suppressorrelay held over ma then be determined from simple formu as in terms ofthe charge of the condenser.

The invention may now be more fully understood by reference to theaccompanying drawings, in which Figure 1 illustrates a form of testingcircuit for an echo suppressor, Fig. 2 is a curve illustrating theoperation of the arrangement of Fig. 1 and Fig. 3 illustrates thecircuit details of the echo suppressor.

In order to understand the invention the nature of the echo suppressorto which the invention is applied Wlll first be described. Referring toFig. 3, the echo suppressor arrangement comprises a vacuum tubeamplifier A working into a vacuum tube detector D in the output circuitof which is a detector relay DR. The inputterminals 12 of thearrangement are in practice bridged across one line ofafour-wire circuitso that voice currents transmitted over said circuit will be impressedupon the amplifier. These voice currents are amplified by the amplifierA and are rectified by the detector D, whose grid has such a potentialthat normally no current will flow through the detector relay, but whenthe voice currents are received a rectified current component is causedto flow through the detector relay to actuate the same. The detectorrelay DR controls the circuits of a hangover relay HR and a suppressorrelay SE in such a manner as to energize both relays. The hangover relaycloses a locking circuit through an additional winding of the suppressorrelay SR. so that the latter will remain locked up so' long as thehangover relay HR remains energized. The latter relay is made slowreleasing by means of an additional short. circuiting winding and thetime required for the hangover relay to release may be adjusted by meansof a suitable resistance in circuit with the extra winding.Consequently, after the voice-currents cease and the detector relay DRbecomes deenergized a definite interval of time must elapse before thesuppressor relay SR releases its contacts, this interval of time beingdetermined by the hangover rela which in turn is regulated by means of te resistance, already referred to.

The result is that the arrangement as a whole causes the suppressorrelay SR to close its contacts very quickly when voice currents areapplied to the terminals 12, but a definite interval of time must elapseafter the voice currents cease before the suppressor relay SR willrelase its armature. In order to determine the time required for thesuppressor relay to release its armature after the alternating currenthas ceased to be applied to thesuppressor the circuit arrangement ofFig. 1 may be utihzed. In Fig. 1, a source of alternating current, suchas an oscillator S, is connected through a system of resistances 2 tothe input terminals 12 of the echo suppressor. The source S generates analternating current which, for purposes of the test, will be equivalentto a voice current, and the resistances are roportioned to give asuitable voltage value or applications to the term1nals of the echosuppressor. A switch t, with two make contacts, has one of its contacts,5, bridged across the input terminals of the echo suppressor so as toshort circuit it when the key is operated. The other contact, 8, isincluded in the test circuit, to be described later. This switch ispreferably so consructed that it closes with a snap action in a mannerwell known in the art and the two contacts close within a negligiblysmall fraction of a second of each other. The closure of the contact 5interrupts the application of alternating current to the suppressor andstarts the releasing action which culminates in the release of thesuppressor relay SE.

The closure of the contact 6 completes a 'circuit from the left-handterminal of the up according to the well known exponential law,gradually approaching the voltage E of the battery 7 as shown by thecurve 30 of Fig. 2. The resistance 8 and the condenser 10 are soproportioned that the voltage of the condenser will not have reached asteady value by the time the suppressor relay contact 9 opens andconsequently the opening of the contact 9 of the suppressor relay SRleaves the condenser charged to some voltage lower than E, dependingupon the time elapsing between the instant the alternating current isinterrupted and the instant that the contact 9 opens. It will beapparent that if the voltage to which the condenser is charged isdetermined, the time required for the echo suppressor to release may beat once determined.

To measure the voltage of the condenser 10, a potentiometer 11 isconnected around the battery 7 so that a portion 6 of its voltage isimpressed upon another condenser 3, which is connected through atelephone receiver 13 to aback contact 14 of the suppressor relay.Obviously, if the condenser 3 is charged to the same potential as thecondenser 10, at the instant the armature of the suppressor relay SRreleases and closes its back contact 14, no sound will be heard in thereceiver 13. On' the other hand, if the potentials to which the twocondensers are charged are not the same a click will be heard, due tothe flow of an equalizing current from one condenser to the other. Byrepeatedly operating the switch 4 and adjusting the potentiometer 11until no click is heard the potential, to which the condenser 3 ischarged, may be made equal to the potential built up in the condenser10. Since when no click is heard this potential will be the potential towhich the condenser 10 is charged, the ratio of e to E can be found fromthe setting of the potentiometer 11, so that the time in seconds may bereadily computed. It should be noted that a back contact 15 is providedon the switch t and establishes a connection from the right-handterminal of the battery 7 over the front contact 9 of the suppressorrelay SE to the condenser 10. This insures that the condenser will becompletely discharged at the start of any measurement.

By giving the resistance 8 and the condenser 10 definite values, thepotentiometer 11 may be designed to indicate time in any convenientunits by means of a pointer moving with its contact and playing over ascale. It is desirable that e be not too close to E, as the test becomesless sensitive and accurate. The time required for the condenser tocharge up to a given fraction of the voltage E can be increased byincreasing the resistance 8 or the capacity of the condenser 10, whichmakes it possible to adjust the constant by which the time scale in thepotentiometer should be multiplied to any desired value.

For this test it is important that the condenser 10 be of good qualityand that it and the associated circuit through resistance 8 and overcontacts 6 and 9 should be free from leakage. 'The condenser 3 is not soimportant, it being merely necessary to have the insulation good enoughto avoid a leak age current, which would disturb the voltage on thecontact of the potentiometer. If the resistance of the potentiometer ismoderate this requirement will not be severe.

The relation between the hangover time of the echo suppressor and thevoltage to which the condenser is charged may be obtained from thefollowing equation:

C e R in which 2? is the time in seconds, R is the resistance in ohms, Gis the capacity of the condenser 10 in farads, and e is the base of thenatural logarithms.

The ratio fiis also the ratio of the resistance of that part of thepotentiometer 11 that is in parallel with the condenser 3 to the totalresistance of the potentiometer. From this relation can be obtained theresistance at which the potentiometer contact must be set to give anydesired time t.

It will also be obvious that the general principles herein disclosed maybe embodied in many other organizations widely different from thoseillustrated Without departing from the spirit of the invention asdefined in the following claims.

What is claimed is:

1. The method of testing the operating time of a translating arrangementincluding a relay responsive to electric current, which consists inapplying current to the input terminals of the translating arrangementfor a period of time, and gradually building up a potential during thetime that it takes for the translating arrangement to respond to theconditions of current and no current at its input.

2. The method of testing the operating time of a translating arrangementincluding a relay responsive to electric current, which consists inapplying current to the input terminals of the translating arangementfor a period of time, gradually building up a potential during the timethat it takes for the translating arrangement to respond to theconditions of current and no current at its input, and balancing thepotential thus built up against a variable known potential.

3. The method of testing the hangover tnne of a translating arrangementincluding a relay responsive to electric current, which consists inapplying current to the input terminals of the translating arrangement,interrupting the current thus applied, and gradually building up apotential from the instant that the interruption occurs until thetranslating arrangement responds to the interruption.

4. The method of testing the hangover time of a translating arrangementincluding a relay responsive to electric current, which consists inapplying current to the input terminals of the translating arrangement,interrupting the current thus applied, gradually building up a potentialfrom the instant that the interruption occurs until the translatingarrangement responds to the interruption, and balancing the potentialthus built up against a variable known potential.

5. A; system for testing the operating time of a translating arrangementincluding a relay responsive to electric current, said-system comprisingmeans for applying and then interrupting the application of current tothe input terminals of the translating arrangement, a condenser, andmeans to charge said condenser during the time that the translatingarrangement utilizes in responding to a change in the current conditionat its input terminals.

6. A system for testing the operating time of a translating arrangementincluding a relay responsive to electric current, said system comprisingmeans for applying and then interrupting the application of current tothe input terminals of the translating arrangement, a condenser, meansto charge said condenser during the time that the translatingarrangement utilizes in respond ing to a change in the current conditionat its input terminals, and means to balance the potential to which thecondenser is charged against a known and variable potential.

7. A system for testing the hangover time of a translating arrangementincluding a relay responsive to electric current, said system comprisingmeans for applying and then interrupting the application of current tothe input terminals of the translating arrangement, a condenser, meanswhereby the condenser will begin to charge up at the instant thealternating current is interrupted,

and means to prevent further charge of the condenser when thetranslating arrangement ceases to operate after the alternating currenthas been interrupted.

8. A system for testing the hangover time of a translating arrangementincluding a relay responsive to electric current, said system comprisingmeans for applying and then interrupting the application of current tothe input terminals of the translating arrangement, a condenser, meanswhereby the condenser will begin to charge up at the instant the currentis interrupted, means to prevent further charge of the condenser whenthe translating arrangement ceases to operate after the current has beeninterrupted, and means to balance the potential to which the condenserhas been charged against a known and variable potential.

9. A system for testing the hangover time of a translating arrangementincluding a relay responsive to electric current, said system comprisingmeans for applying and then interrupting the application of current tothe input terminals of the translating arrangement, a condenser, meanswhereby the condenser will begin to charge up at the instant the currentis interrupted, means to prevent further charg of the condenser when thetranslating arran ement ceases to operate after the current has beeninterrupted, a second condenser, a potentiometer and circuit connectionstherefor whereby said second condenser may he charged to any desiredknown otential, and means to balance the potential to which said firstmentioned condenser was charged against the 10 charge of said secondmentioned condenser. In testimony whereof, I have si ned my name to thisspecification this 22nd day of October 1923.

GEORGE GRISSON.

